This is a fine structural study of the choroid body of teleost eyes. This structure, sometimes called a gland, is a rete mirabile composed of interdigitating afferent and efferent capillaries in the form of a counter-current system. Species of fish that have well developed choroid bodies have elevated oxygen tensions in the retina (cod 820mm. average 1,180mm. maximum observed). Fish having no choroid body have low tensions (eel 18mm. average). Those with small choroid bodies have intermediate pressures (sea bass 161mm., tautoug 210mm. average). Elevated concentrations of cytochrome C has been found in the eyes. There may be a functional relationship between the pseudobranch gland (which is in the direct circulation to the eye) and the choroid body. The pseudobranch produces tremendous levels of carbonic anhydrase inhibitor, Diamox, completely suppresses the oxygen elevation in the eye. Fish also possess another rete mirabile, the "red gland" of the swim bladder which is involved in a similar function, elevation of gas pressure (primarily oxygen) in the lumen of the swim bladder. Comparisons are being made between the systems.